Sextant App

[Benz]

Originally Posted by ChrisOwens

I've only done a celestial fix a couple of times, but I found getting an accurate shot to be a lot harder in practice than it would seem in theory. An old graybeard taught me this: Take five or six shots a couple of minutes apart, even if you aren't trying to do a noon sight. Go below, and, on a piece of graph paper, plot the elevation on the Y axis and the time on the X axis. Eyeball the dots, ignore the ones that are obviously way out of line, and draw a smooth curve to fit them. Now, you don't have to use any specific one of your shots; you can take any point along that line and plug it into the math as though it was an actual shot.

Seems reasonable, thanks. I like to shoot and reduce several sights one after another if I'm really trying to pinpoint things--the plotted LOPs average out nicely, and outliers are soon detected.

[SeanPatrick]

BTW, the best app [well, suite of apps actually] that I know of for celestial navigation is Navigational Algorithms. I have no affiliation with the developer (Andres Ruiz González) or his software, other than having conversed with him on NavList.

The software is available for Android and Windows. The windows version integrates well with OpenCPN - celestial LOPs can be plotted directly on the chart. I'm not sure if the mobile version has the same functionality. Andres has regularly updated the software in the past to keep it accurate and I have no reason to believe he will not continue to do so.

Cheers!

[Jim Cate]

Years ago, back when I was relying upon celestial I found and used a DOS based program called PCNAV (IIRC). It lived on a single floppy, would reduce sights, average multiple sights, had a star finder for use in broken cloud and output good LOPs... a very versatile and useful bit of software. I still have the disc, but no computer that can access it.

That's progress!

Jim

[HankOnthewater]

For original poster: I knew of a calculator (~30 years ago) that did (some) of the calculations. It was made by Sharp I think and model name was "Merlin".
On Edit: yes it was Sharp, see also here: https://www.cruisersforum.com/forums...uter-8820.html

For SeanPatrick: I would be very interested to see the Excel spreadsheet.

[double u]

I never could comprehend, why one would substitute the by-far easiest part of celestial navigation, the "calculations", with an electronic device+procedure. I could not comprehend it in the days when we were obliged to use celestial (our 1st & 2nd rtw), & I can understand it less today, when one would learn & practice celestial either for old times sakes or out of interest. If you want to make it easy for yourself - why bother?
It is just a matter of selfconfidence (or the lack of it).
The part that can be substituted by apps/calculators/programs,.. is by far the easiest the "math" is simple adding & substracting numbers that one has picked out of 2 books.
Getting sights to a repeatable accuracy of, say, 3 miles will require exercise & application, lots of it. To reliably find an offshore reef with hardly anything showing above water would require more accuracy in calm weather with little breaks.
Somebody with the stamina to familiarise him/herself & practice with the sextant until an acceptable accuracy is attained will certainly be intelligent & studious enough to master the "math"!
Just a matter of self confidence...

[svRaison d Etre]

Originally Posted by double u

I never could comprehend, why one would substitute the by-far easiest part of celestial navigation, the "calculations", with an electronic device+procedure. I could not comprehend it in the days when we were obliged to use celestial (our 1st & 2nd rtw),

I do not have too much experience with celestial navigation, but I remember that I had to carry a lot of sight reduction book or Almanacs that you could extrapolate for a year or so 30 years ago. I still have that floppy disk that came with the Davis sextan.

So a app would be great, keep a old cellphone in faraday box and if lighting strike dust of the old sextant

[double u]

as illustration my paperwork from an evening round of starsights with Selected Stars

[double u]

4 book cover you:
Nautical Almanac
3 x Sight reduction tables (0-39° Lat, 40-89°Lat, Selected Stars)

[SVTatia]

Originally Posted by HankOnthewater

For original poster: I knew of a calculator (~30 years ago) that did (some) of the calculations. It was made by Sharp I think and model name was "Merlin".
On Edit: yes it was Sharp, see also here: https://www.cruisersforum.com/forums...uter-8820.html

For SeanPatrick: I would be very interested to see the Excel spreadsheet.

I think you are referring to Celesticomp V? Still available on eBay.

https://oceannavigator.com/navigatin...tial-computer/

[double u]

the plotting of the above:

[double u]

& here "real" celestial...

[double u]

Originally Posted by svRaison d Etre

... if lighting strike dust of the old sextant

if you do not practice at least twice weekly, you will be so far out that you will not know if the island on the horizon is Hawaii or Tahiti...

[SeanPatrick]

Originally Posted by HankOnthewater

For SeanPatrick: I would be very interested to see the Excel spreadsheet.

Link is in my signature*. I have an update in the works which will add the option to display the celestial equator and ecliptic, as well as the ability to increment/decrement the almanac time in steps of days, hours, minutes or seconds. I have also updated the delta-T values, although the values in the current version are still extremely accurate.

The new version is essentially ready, I just have to update the pictures, description and notes on my website. But if anyone really wants to see it right now, I can PM a link to the new version.

Cheers!

*Oddly, my signature didn't appear in the post mentioning the spreadsheet. I suppose it's because I made that post on my mobile phone?

[Jim Cate]

Originally Posted by double u

I never could comprehend, why one would substitute the by-far easiest part of celestial navigation, the "calculations", with an electronic device+procedure. I could not comprehend it in the days when we were obliged to use celestial (our 1st & 2nd rtw), & I can understand it less today, when one would learn & practice celestial either for old times sakes or out of interest. If you want to make it easy for yourself - why bother?
It is just a matter of selfconfidence (or the lack of it).
The part that can be substituted by apps/calculators/programs,.. is by far the easiest the "math" is simple adding & substracting numbers that one has picked out of 2 books.
Getting sights to a repeatable accuracy of, say, 3 miles will require exercise & application, lots of it. To reliably find an offshore reef with hardly anything showing above water would require more accuracy in calm weather with little breaks.
Somebody with the stamina to familiarise him/herself & practice with the sextant until an acceptable accuracy is attained will certainly be intelligent & studious enough to master the "math"!
Just a matter of self confidence...

UU, sounds like you had a good deal more time with celestial than I did, and those star plots look very good indeed. However, I managed several long passages with CN, including one through the Tuamotus (sp?) and always arrived where I was heading.

In my case, the addition of electronic calculators was a big benefit. First, I found t hat the bookwork was likely to induce sea sickness in me if there was much motion on. Second, especially when tired, I made arithmetical and look-up errors that were hard to track down. Third, it was far faster than hand reductions. Fourth, in the later computer editions, it was easy to take several shots and first eliminate outliers and then average the good ones, increasing (for me) the accuracy of the plot. And finally, the "star finder" feature in PCNAV made snap shots through a hole in the clouds a potentially useful technique for me. Of course I carried the necessary books and tools to do manual reduction and plotting and maintained the skills to use them.

I was not using CN out of a sense of romanticism or nostalgia, but rather because that was the best tool for navigation available to me at the time (1983-87), and w hen satellite navigation became available I eagerly adopted it... but still carried all the stuff to do it the "old way" should the new one fail.

For those who have not done CN in anger, as UU says the hard part isn't the sight reduction, but rather making the observation accurately. When your height of eye is only 8 or 10 feet and there is a sea running, the horizon is poorly defined and of course, the boat is moving about vigorously. Deciding when the horizon is touched is kinda guesswork, and lots of experience helps get it right... as does the option to take and reduce lots of sights.

Jim

[st0nes]

A sextant sight does not give you lat/long. It gives you an intercept from an assumed position which lets you draw a line of position (LOP) at right angles to the azimuth of the body.

Yes, there are android apps that will do the grunt work of reducing the sight and give you the intercept, but I had a (brief) look a few months ago and they didn't seem very impressive. If you use a pencil and calculator, and you are willing to put in the time to achieve basic proficiency, you can probably do it quicker manually.